Transcriptional targeting to brain cells: Engineering cell type-specific promoter containing cassettes for enhanced transgene expression

Transcriptional targeting using a mammalian cellular promoter to restrict transgene expression to target cells is often desirable for gene therapy. This strategy is, however, hindered by relatively weak activity of some cellular promoters, which may lead to low levels of gene expression, thus declining therapeutic efficacy. Here we outline the advances accomplished in the area of transcriptional targeting to brain cells, with a particular focus on engineering gene cassettes to augment cell type-specific expression. Among the effective approaches that improve gene expression while retaining promoter specificity are promoter engineering to change authentic sequences of a cellular promoter and the combined use of a native cellular promoter and other cis-acting elements. Success in achieving high level and sustained transgene expression only in the cell types of interest would be of importance in allowing gene therapy to have its impact on patient treatment.

Phospholipid-based artificial viruses assembled by multivalent cations

Self-assembled DNA delivery systems based on cationic lipids are simple to produce and weakly hazardous in comparison with viral vectors, but possess a significant toxicity at high doses. Phospholipids are in contrast intrinsically safe; yet their association with DNA is problematic because of unfavorable electrostatic interactions. We achieve the phospholipid-DNA complexation through the like-charge attraction induced by cations. Monovalent cations are inappropriate due to their poor binding affinity with lipids as inferred from electrophoretic mobility, whereas x-ray diffractions reveal that with multivalent cations, DNA is complexed within an inverted hexagonal liquid-crystalline phase. Coarse-grained Monte Carlo simulations confirm the self-assembly of a DNA rod wrapped into a lipid layer with cations in between acting as molecular glue. Transfection experiments performed with Ca2+ and La3+ demonstrate efficiencies surpassing those obtained with optimized cationic DOTAP-based systems, while preserving the viability of cells. Inspired by bacteriophages that resort to polycations to compact their genetic materials, complexes assembled with tetravalent spermine achieve unprecedented transfection efficiencies for phospholipids. Influence of complex growth time, lipid/DNA mass ratio, and ion concentration are examined. These complexes may initiate new developments for nontoxic gene delivery and fundamental studies of biological self-assembly.

GAL4-NF-kappaB fusion protein augments transgene expression from neuronal promoters in the rat brain

Targeted gene expression mediated by a mammalian cellular promoter is desirable for gene therapy in the brain, where there are a variety of different neuronal phenotypes, several types of supportive cells, and blood vessels. However, this approach can be hampered by weak activity of some cellular promoters. In view of the potency of the transcription factor NF-kappaB in regulating neuronal gene expression, we have assessed whether it can be used to enhance the strength of neuron-specific promoters. Our approach was to use a neuronal promoter to drive expression of a chimeric transactivator, which consisted of a part of the transcriptional activation domain of the NF-kappaB p65 protein fused to the DNA-binding domain of GAL4 protein from yeast. The second copy of the neuronal promoter was modified by introducing the unique GAL4 binding sequences at its 5' end and used to drive the expression of a transgene. Binding of the chimeric transcriptional activator upstream of the second promoter was expected to potentiate its transcriptional activity. In this study, the approach was applied to the platelet-derived growth factor beta chain and synapsin-1 neuron-specific promoters and tested in vitro and in vivo using plasmid, lentiviral, and baculoviral vectors. We observed up to a 100-fold improvement in reporter gene expression in cultured neurons and 20-fold improvement in the rat brain in vivo. Moreover, the cell-type specificity of the two tested promoters was well preserved and restricted to neurons. Finally, the expression driven by the new lentiviral vectors with the p65-potentiated synapsin-1 promoter showed no signs of decline or cell damage 4 weeks after injection. This approach should be suitable for constructing powerful and stable gene expression systems based on weak cell-specific promoters in neuronal phenotypes.

Role of Polo-like kinase in the degradation of early mitotic inhibitor 1, a regulator of the anaphase promoting complex/cyclosome

Early mitotic inhibitor 1 (Emi1) inhibits the activity of the anaphase promoting complex/cyclosome (APC/C), which is a multisubunit ubiquitin ligase that targets mitotic regulators for degradation in exit from mitosis. Levels of Emi1 oscillate in the cell cycle: it accumulates in the S phase and is rapidly degraded in prometaphase. The degradation of Emi1 in early mitosis is necessary for the activation of APC/C in late mitosis. Previous studies have shown that Emi1 is targeted for degradation in mitosis by a Skp1-Cullin1 F-box protein (SCF) ubiquitin ligase complex that contains the F-box protein beta-TrCP. As with other substrates of SCF(beta-TrCP), the phosphorylation of Emi1 on a DSGxxS sequence is required for this process. However, the protein kinase(s) involved has not been identified. We find that Polo-like kinase 1 (Plk1), a protein kinase that accumulates in mitosis, markedly stimulates the ligation of Emi1 to ubiquitin by purified SCF(beta-TrCP). Cdk1-cyclin B, another major mitotic protein kinase, has no influence on this process by itself but stimulates the action of Plk1 at low, physiological concentrations. Plk1 phosphorylates serine residues in the DSGxxS sequence of Emi1, as suggested by the reduced phosphorylation of a derivative in which the two serines were mutated to nonphosphorylatable amino acids. Transfection with an small interfering RNA duplex directed against Plk1 caused the accumulation of Emi1 in mitotically arrested HeLa cells. It is suggested that phosphorylation of Emi1 by Plk1 is involved in its degradation in mitosis.

UV irradiation triggers ubiquitin-dependent degradation of p21(WAF1) to promote DNA repair

p53-mediated increase in cyclin-dependent kinase inhibitor p21(WAF1) protein is thought to be the major mediator of cell cycle arrest after DNA damage. Previously p21 protein levels have been reported to increase or to decrease after UV irradiation. We show that p21 protein is degraded after irradiation of a variety of cell types with low but not high doses of UV. Cell cycle arrest occurs despite p21 degradation via Tyr(15) inhibitory phosphorylation of cdk2 and differs from the classical p21-dependent checkpoint elicited by ionizing radiation. In contrast to the basal turnover of p21, degradation of p21 switches to ubiquitin/Skp2-dependent proteasome pathway following UV irradiation. ATR activation after UV irradiation is essential for signaling p21 degradation. Finally, UV-induced p21 degradation is essential for optimal DNA repair. These results provide novel insight into regulation of p21 protein and its role in the cellular response to DNA damage.

Control of meiotic and mitotic progression by the F box protein beta-Trcp1 in vivo

SCF ubiquitin ligases, composed of three major subunits, Skp1, Cul1, and one of many F box proteins (Fbps), control the proteolysis of important cellular regulators. We have inactivated the gene encoding the Fbp beta-Trcp1 in mice. beta-Trcp1(-/-) males show reduced fertility correlating with an accumulation of methaphase I spermatocytes. beta-Trcp1(-/-) MEFs display a lengthened mitosis, centrosome overduplication, multipolar metaphase spindles, and misaligned chromosomes. Furthermore, cyclin A, cyclin B, and Emi1, an inhibitor of the anaphase promoting complex, are stabilized in mitotic beta-Trcp1(-/-) MEFs. Indeed, we demonstrate that Emi1 is a bona fide substrate of beta-Trcp1. In contrast, stabilization of beta-catenin and IkappaBalpha, two previously reported beta-Trcp1 substrates, does not occur in the absence of beta-Trcp1 and instead requires the additional silencing of beta-Trcp2 by siRNA. Thus, beta-Trcp1 regulates the timely order of meiotic and mitotic events.

The functions of the cdk-cyclin kinase inhibitor p21WAF1

p21WAF1 plays a critical role in regulating cell growth and the cell response to DNA damage. The primary targets of p21WAF1 (hereafter referred to as p21) are the cdk-cyclins which regulate the progression of eukaryotic cells through the cell cycle, and proliferating cell nuclear antigen (PCNA), an accessory protein of DNA polymerase delta. p21 forms complexes with a class of cdk-cyclins to inhibit their kinase activity and with PCNA to inhibit DNA synthesis. These distinct properties map to the N-terminal and the C-terminal regions of p21, respectively. Cell cycle arrest in G-1 (G-1 checkpoint) following DNA damage is mediated by p53 and is deficient in p21 null cells. p53 thus upregulates p21 expression in response to DNA damage, which in turn inhibits cdk2-associated kinase activity. Retinoblastoma protein is regulated by cdk-cyclin kinases, and acts as a downstream target of p21 in DNA damage-induced G-1 arrest. Furthermore, accumulating evidence indicates that p21 may play a role in maintaining G-2 arrest after DNA damage. Transcriptional control of p21 by factors other than p53 is critical for growth arrest and for cell differentiation in many instances.

Growth inhibition by CDK-cyclin and PCNA binding domains of p21 occurs by distinct mechanisms and is regulated by ubiquitin-proteasome pathway

The CDK inhibitor, p21WAF1/Cip1 blocks cell cycle progression. In vitro, the N-terminus of p21 binds and inhibits CDK-cyclin kinase activity, whereas the C-terminus binds and inhibits PCNA (proliferating cell nuclear antigen) function. PCNA is essential for processivity of both DNA polymerase delta and epsilon. We have performed a detailed analysis of growth inhibition by the N- and C-terminal regions of p21, and determined whether the N- and C-terminal regions mediate this effect by different mechanisms. Expression of either the N- or the C-terminal region of p21 inhibits DNA synthesis and cell growth, but not as efficiently as full length p21. The effectiveness of the two p21 domains is dependent on their stability which is determined by the ubiquitin-proteasome pathway. The stabilization of the N- and C-terminal region of p21 increases their effectiveness as inhibitors of DNA synthesis to levels comparable to full length p21. Inhibition of DNA synthesis by the N-terminal region of p21 involves suppression of E2F activity. In contrast, inhibition by the C-terminal region of p21 is not accompanied by suppression of E2F activity, but is mediated via PCNA binding. The C-terminal region of p21 therefore inhibits cell growth by a mechanism distinct from that of the N-terminal region containing the CDK-cyclin inhibitory domain.

Growth inhibition by CDK-cyclin and PCNA binding domains of p21 occurs by distinct mechanisms and is regulated by ubiquitin-proteasome pathway

The CDK inhibitor, p21(WAF1/Cip1) blocks cell cycle progression. In vitro, the N-terminus of p21 binds and inhibits CDK-cyclin kinase activity, whereas the C-terminus binds and inhibits PCNA (proliferating cell nuclear antigen) function. PCNA is essential for processivity of both DNA polymerase delta and epsilon. We have performed a detailed analysis of growth inhibition by the N- and C-terminal regions of p21, and determined whether the N- and C-terminal regions mediate this effect by different mechanisms. Expression of either the N- or the C-terminal region of p21 inhibits DNA synthesis and cell growth, but not as efficiently as full length p21. The effectiveness of the two p21 domains is dependent on their stability which is determined by the ubiquitin-proteasome pathway. The stabilization of the N- and C-terminal region of p21 increases their effectiveness as inhibitors of DNA synthesis to levels comparable to full length p21. Inhibition of DNA synthesis by the N-terminal region of p21 involves suppression of E2F activity. In contrast, inhibition by the C-terminal region of p21 is not accompanied by suppression of E2F activity, but is mediated via PCNA binding. The C-terminal region of p21 therefore inhibits cell growth by a mechanism distinct from that of the N-terminal region containing the CDK-cyclin inhibitory domain.